Traditionally, the normal way of producing a melt for slag, stone or rock fibres has been by means of a shaft furnace in which a self-supporting stack of inorganic particulate material is heated by combustion of combustible material in the furnace. The stack gradually melts and is replenished from the top, with melt draining down the stack and out from the bottom of the furnace. The normal furnace for this purpose is a cupola furnace.
It is necessary for the stack to be self-supporting and permeable to the combustion gases, which are generally generated by combustion of carbonaceous material in the stack. It is therefore necessary that everything in the stack is relatively coarse (in order that the stack is permeable) and has high physical strength and does not collapse until combustion or melting is well advanced. In practice this means that the carbonaceous material is coke and the particulate material is either coarsely crushed rock, stone or slag.
If fine particles of mineral material such as waste mineral wool are used it is necessary to incur the expense and inconvenience of forming it into briquettes. Briquetting usually uses sulphur-containing materials as binder, such as Portland cement with gypsum, and this means that the effluent is liable to have a high sulphur content, which has to be treated.
The cupola or other stack furnace system also has the disadvantage that conditions in the furnace always tend to be sufficiently reducing that some of the iron is reduced to metallic iron. This necessitates separating metallic iron from the melt, reduces wool production, leads to the provision of iron waste and also tends to incur the risk of corrosion in the section containing iron and slag.
Another disadvantage is that the process does not have high thermal efficiency.
Despite these disadvantages, the process using a cupola or other stack furnace has been widely adopted throughout the world for the manufacture of rock, stone or slag fibres.
An alternative and entirely different system for the production of a mineral melt that avoids or reduces the disadvantages of the cupola system is disclosed in our earlier publication WO 03/002469. This system involves suspending powdered coal, or other fuel, in preheated combustion air and combusting the suspended fuel in the presence of suspended particulate mineral material in a circulating combustion chamber, i.e., a combustion chamber in which the suspended particulate materials and air circulate in a system which is or approaches a cyclone circulation system. This is commonly referred to as a cyclone furnace.
The suspension of coal in preheated air, and the particulate mineral material, are introduced through the top or close to the top of the combustion chamber. Within the combustion chamber, combustion of the particulate coal occurs and the particulate material is converted to melt. The melt and particulate material that is not yet melted is thrown onto the walls of the chamber by the circulating gases and will flow down the chamber. The melt is collected in a settling tank at the bottom of the chamber.
In order to increase the energy efficiency of the cyclone furnace in WO03/002469, the exhaust gases, which leave the circulating chamber at a temperature in the range of 1400 to 1700° C., are used to preheat the particulate material. WO 03/002469 teaches that the exhaust gases are quenched to 1000 to 1500° C. and then mixed with the mineral material to preheat it to a temperature of 700 to 1050° C.
EP-A-1889876 and WO 2008/019780 also disclose a cyclone system.
The cyclone furnace has significant advantages compared to cupola or other stack furnaces. With respect to fuel, it avoids the need for briquetting fine particles and a wide range of fuels can be used including, for example, plastic. Using a melting cyclone furnace eliminates the risk of reduction of the ores to iron and releases exhaust gases which are environmentally acceptable. The flexibility in melt capacity is much better than with a cupola furnace meaning that production can easily and quickly be switched, from, for example, 40% to 100% of total capacity so the time taken to respond to changing demands is greatly reduced. Furthermore, melting in a cyclone furnace is much quicker than is the case for a cupola furnace and is in the order of minutes, rather than in the order of hours.
Hence, using a melting cyclone furnace system is economically and environmentally desirable and the system disclosed in WO 03/002469 works well. There is, however, room for improvement in the process.
In WO 03/002469 the mineral material preferably includes an unspecified proportion of waste bonded mineral wool. It is generally beneficial to be able to recycle a waste material. However, the inventors have discovered that when bonded mineral wool is used in the system of WO 03/002469 there is a tendency for the mineral material to lose its free-flowing particulate characteristics and become sticky. This is particularly the case when a significant amount of waste mineral wool is used, such as 5% or more of the total mineral material.
The loss of free-flowing characteristics of the mineral material impedes the efficient flow of the mineral material and the gases in the heat exchange system and can even lead to this becoming blocked. It also reduces the efficiency of the combustion in the circulating combustion chamber as more energy is required to melt large mineral material agglomerates than is required to melt more finely divided particles.
The object of the present invention is to provide a method of making mineral wool which can be used to recycle waste mineral material while maintaining the flow properties of the mineral material and achieving a high level of energy efficiency.
U.S. Pat. No. 5,006,141 describes a method for production of glass using combustion heat to melt glass making material in a glass making furnace. The furnace used is not a circulating combustion chamber furnace. Two feedstocks are used to produce the melt, one being a batch feedstock and one being glass cullet. The cullet is preheated before the batch feedstock. There is no teaching that the batch feedstock has lower sintering temperature than the cullet. The cullet is preheated to about 650° C. (1200° F.) and the batch feedstock is preheated to a temperature of about 250° C. (about 490° F.).